Quizalofop-p-ethyl which is useful as an active ingredient for a herbicide, is known to have two types of crystal forms, i.e. low melting point crystals (hereinafter referred to as α-crystals) and β-crystals. JP-B-4-76721 discloses that β-crystals can be produced by gradually cooling a solution having quizalofop-p-ethyl dissolved in a solvent, with stirring and maintaining it at a crystallizing temperature, and the produced β-crystals are ones having crystallites of at most 1 μm agglomerated, whereby in some cases, transfer of the solvent slurry, or filtration and drying of the precipitated crystals, are difficult.